Have you tried this for NextGen? – http://wordpress.org/support/topic/nextgen-image-gallery-captions
Excerpt (many other useful tips at the linked thread):
ENABLING CAPTIONS BELOW GALLERY IMAGES
When you add a gallery to a post/page, you get this by default:
[nggallery id=1]You need to add this:
[nggallery id=1 template=caption]GETTING THE CAPTIONS YOU JUST ENABLED TO SHOW-UP
When importing an image to a gallery, NextGEN automatically adds the
image filename (minus the ext) to the ALT/TITLE field of the image
meta data.However, the “template=caption” function is actually preset to call
the DESCRIPTION field of the image. If you don’t have anything in the
DESC field, you will not see any caption (even though it is
technically working.)You have two options: (a) add something to the description field of
every image; or (b) change which field is being called.You can add a value to image DESC fields manually in NextGEN or
(probably) using any number of EXIF tools available.However, suppose you want the image name to automatically appear as
the image caption in your gallery. (Maybe you are creating a product
gallery where the image names are also the actual product codes, and
you want to display those codes below each thumbnail.) No problem –Since NG automatically creates the ALT/TITLE from the image name on
import, here’s how to call that field:
- In your dashboard, click Pluginns > Editor from the left-hand nav
- Select plugin to edit: NextGEN Gallery
- From right hand file list, click: nextgen-gallery/view/gallery-caption.php
- Scroll down to this line: php echo $image->caption
- Change to: php echo $image->alttext
Now, your default image galleries should automatically show the
filename (minus ext) as the caption below every image.